In the construction of window units or assemblies using extrusions of plastics materials such as polyvinylchloride (PVC), for example, as disclosed in U.S. Pat. No. 4,941,288, U.S. Pat. No. 5,003,747, U.S. Pat. No. 6,055,782, U.S. Pat. No. 6,826,871 and U.S. Pat. No. 7,975,432 which issued to the inventor and assignee of the present invention, it has been found desirable to provide an operable window unit or assembly which significantly increases thermal conductivity resistance, windload resistance, storm-driven debris impact resistance and also an increased barrier to sound transmission. It has also been found desirable to provide a window assembly with increased air and water infiltration resistance, forced entry resistance, and an increased protection from infrared and ultraviolet light. Furthermore, it is desirable to provide all of these desirable features in a window unit or assembly that is convenient to use as well as economical in construction. While operable window assemblies have been produced or proposed that provide some of the above features, none of the assemblies provides all of the desirable features and advantages mentioned above.